Grass
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There are 7 subfamilies: |
The grass family, Poaceae, is one of the largest plant families, and to us humans perhaps the most important. There are about 600 genera and perhaps 10,000 species of grasses. It is estimated grasslands comprise 20% of the vegetation cover of the earth.
What is Grass?
Grasses, like orchids and palms, are monocotyledonous, having only one cotyledon, or embryo leaf, rather than the two found in most flowering plants. Most grasses grow low to the ground, although a few, such as corn, sugar cane, and bamboo, are very tall.
Grasses generally have the following characteristics:
- Typically hollow stems (called culms), plugged at intervals (the nodes).
- Leaves, arising at nodes.
- Leaves differentiated into a lower sheath hugging the stem for a distance and a blade; a ligule (a membranous appendage or ring of hairs) lies at the junction between sheath and blade.
- Small, wind-pollinated flowers (called florets) sheathed inside two glumes (bracts), lacking petals, and grouped into spikelets, these arranged in a panicle, raceme, spike, or head.
- Fruit that is a caryopsis, the seed and fruit merged together to form a hard dry grain.
Grass leaves grow from the base of the blade. This enables them to quickly regrow after being eaten by animals, or mowed by a lawn mower. Most grasses send out creeping stems, called stolons if they grow above the ground and rhizomes if they grow below the ground, from which grow new plants. Some grass species are annual, dying at the end of each growing season and leaving seed to grow a new generation the next season, while some are perennial, living for more than one or two years.
Grass in Nature
Most grasses grow in places that are too dry for forests. Grasslands are areas dominated by grass. Prairies are wetter with taller grasses while steppes are dryer with shorter grasses. Savannas are glasslands with widely spaced trees. Large areas of grassland are found in Europe, Asia, Africa, North and South America, and Australia. Many animals depend on the leaves and grains of grasses for food; grasslands support a rich community of living things. Their roots hold the soil in place preventing erosion while their rapid growth allows them to quickly repopulate and recover an area after a fire.
Grass and Humans
Early humans ate the grains of grasses and hunted in grasslands. About 10,000 years ago people in the Middle East started to plant wheat, barley, and other grasses and harvest the grains. Over many generations the plants changed and become domesticated, different from their wild ancestors and dependent on humans for survival. The same thing was happening with rice in the Far East and, a little later, with corn in the New World. While this was happening animals were also being domesticated and soon sheep, goats, and cattle were grazing on wild grasslands and providing food for humans.
Today most human food comes from grasses, directly as grain or indirectly as feed for meat and dairy producing animals. A great part of the world's grasslands have now been converted to human use. In the United States and Canada almost all of the old prairies are now corn and wheat fields. Besides food grasses also provide us with fuel and building materials. They prevent erosion, decorate our gardens, and provide a surface for parks and sports fields.
Wheat
The wheat genus, Triticum, contains about 30 species. Two species of wild wheat, wild einkorn T. boeoticum wild emmer T. dicoccoides, still grow in the Middle East. Wheat is an annual, as are all other grasses grown for grain since they produce more seeds than perennials. It grows about .6 to 1.5 meters (2 to 5 feet) tall. Wheat is mostly grown in areas that are too cold for other crops. China, India, the United States, and Russia are the biggest wheat growing countries. Wheat grains are ground up making flour which is used to make bread, pasta, and other foods. Wheat straw is used to feed animals.
Rice
Almost all the rice grown is Asian rice, Oryza sativa. There is also an African rice, O. glaberrima, which was first domesticated in West Africa about 3500 years ago. Rice grows in a warm, humid climate and needs lots of water. Rice fields, or paddies, are flooded and drained several times during the growing season. About 90 percent of the world's rice crop is grown in Asia where it makes up the largest part of most people's diets. Rice plants stand about 1.2 to 1.8 meters (4 to 6 feet) tall, however never varieties have been breed that are only around 0.9 meters (3 feet) tall in order to prevent them from falling over as the grain matures, which has been a major cause of crop loss.
Corn
Sugar Cane
Bamboo
External links and references
- Kew Index of World Grass Species
- Definitions of Grass structures
- Poaceae in L. Watson and M.J. Dallwitz] (1992 onwards). The families of flowering plants: descriptions, illustrations, identification, information retrieval.
- L. Watson and M. J. Dallwitz (1992 onwards). The grass genera of the world: descriptions, illustrations, identification, and information retrieval; including synonyms, morphology, anatomy, physiology, phytochemistry, cytology, classification, pathogens, world and local distribution, and references.
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